I haven’t checked it personally, but I hear that Messenger.NET is no longer working. Apparently, MSN shut off access to all versions of their protocol prior to MSNP8 (Messenger.NET uses imcli which implements MSNP7) back in October.
BTW, as an alternative, I found the dotMSN library on hypothetic’s project page. Version 1.1 of dotMSN implements MSNP8. It’s free, though the source is not provided. I have no idea if it’s any good, but it’s the only .NET library for MSN messenger that implements MSNP8 that I could find.